Drilling and completing oil and gas wells are highly expensive undertakings as oil and gas bearing formations are generally located many thousand of feet below the surface of the earth. Since the cost of drilling a well is strongly time dependent, the faster the drilling operation is completed, the lower the cost in drilling the well.
Directional drilling techniques are widely known in the drilling industry for drilling oil and gas wells. One commonly used technique uses a hydraulically powered drilling motor to rotate a drill bit. The hydraulic power is provided by drilling fluid pumped down through the drill string from the surface. A “steerable” motor housing commonly includes a small angle bend along its axis (e.g., from about 0.5 to about 3 degrees). The direction of drilling may be controlled by selecting the drilling mode and the tool face angle of the bent housing. In the “rotary drilling” mode, the drill string is rotated at the surface such that the drilling motor rotates with the drill string. Rotary drilling is intended to maintain the current drilling direction (e.g., along the present inclination and azimuth). In “slide drilling” mode, the drill string is not rotated at the surface. Slide drilling is intended to change the drilling direction (i.e., to turn the wellbore) towards the tool face angle of the bent housing.
While such techniques have been commercially serviceable for many years, there are several drawbacks. For example, the toolface angle of the bent housing is commonly communicated to the surface via a low bandwidth mud pulse telemetry signal. Adjusting the toolface angle can therefore be a highly time consuming process. Moreover, slide drilling can be particularly problematic (especially in deep wells) due to static frictional forces between the drill string and the borehole wall. These frictional forces can make it difficult to adjust the toolface angle and to maintain weight on bit during drilling.